Scrap or repair? Advice please

Well it's the devil you know vs. the devil you don't, if you sell this to buy another used car. 2004 is getting pretty long in the tooth, but generally as long as you keep putting parts in them, they keep running. Generally, if you can do most of the work or at least some of the work yourself, owning an older Mercedes can be more economical than purchasing a newer "dependable" car. If you must pay someone else, any older car will probably eat you alive with repair bills, but Mercedes will be worse than some (Japanese cars) but better than others (Audi, BMW).

I'm thinking that if you buy another used Mercedes to replace this, I'd get one that is at least 3 or 4 years old, and one that had an engine and transmission that has a good reputation for dependable service (in other words, the same engine and transmission was used in many other models and had 5 or 7 years of service life in other cars, so that there are a lot of examples out there for determining the dependability).

Regarding the ABS pump, have you checked with the dealer to see if this is covered under a recall, and you can get it fixed for free?

Here in the US the brake system pump for many models was covered by a recall because MB had used defective parts. Have you checked with your local MB dealer to see if the brake repair should be covered under recall?

Another possible cause is flexible brake lines are bad. They can fail internally and become check valves, so they will allow brake fluid pressure to build, but won't allow the pressure to release. If the pressure can be released by opening the bleeder valve, that's your problem.